Harden tests against upcoming `start` changes
Harden tests against upcoming `start` changes

We'll now be reporting errors in sunk `start` blocks; doing so made
these tests fail, since they expected silent discarding of exceptions
inside of `start`. (Note that the semantics only change for 6.d, so
this will not cause breakage for the test as it originally was in 6.c,
if it indeed was part of 6.c.)

perl6/roast,dogbert17/roast,dogbert17/roast